Complementary metal oxide semiconductor (CMOS) devices are dominated by n-channel (NMOS) and p-channel (PMOS) transistor structures. Various physical characteristics of each type of transistor determine the threshold voltage (Vt) which must be overcome to invert the channel region and to cause a given transistor to conduct majority carriers (either by electron movement in an NMOS device or by hole movement in a PMOS device).
One of the controlling physical characteristics of a transistor is the work function of the material used to form the gate electrode of the transistor device. In semiconductor devices such as dynamic random access memory (DRAM) devices, transistor gates are predominantly polysilicon and an overlying layer of metal silicide. However, polysilicon transistor gates are prone to polysilicon depletion effects such as unwanted voltage drops which reduce transistor performance.
A second problem prone to boron-doped polysilicon gate is boron penetration. Boron penetration results from migration of p-type dopants from a polysilicon gate of a p-channel transistor through the gate oxide region into the transistor channel. A common remedy is to incorporate nitrogen into the gate oxide. While nitrogen decreases boron penetration in p-channel transistors, it has undesirable effects on n-channel transistors such as decreased carrier mobility. However, the benefit/cost effects of global nitrogen incorporation generally outweigh the adverse effects, and thus nitrogen incorporation is performed globally.
Metal gate technology presents an alternate approach for CMOS transistor devices, as the traditional polysilicon gate electrodes are replaced with metal or metal alloy electrodes. To substitute the traditional p-type or n-type polysilicon transistor gate with a metal or metal compound, it is desirable to use metals which have similar work function characteristics of p-type or n-type polysilicon to obtain a comparable transistor threshold voltage. N-type doped silicon has a work function of approximately 4.15 electron volts (eV). Metals having a similar work function include aluminum, manganese, zirconium, niobium, hafnium, and tantalum. P-type doped silicon has a work function of approximately 5.2 eV. Metals having a similar work function include nickel, cobalt, platinum, and ruthenium. A method and structure for CMOS transistor gates of metal are described in US Patent Application Publication 2004/0256679 by Yongjun J. Hu, assigned to Micron Technology, Inc. and incorporated herein by reference as if set for in its entirety.
The manufacturability of CMOS using dual metal to engineer the work function is not trivial and leads to myriad complexities in the integration. A simpler alternative would be to have a dual work function scheme which is compatible with the current CMOS flow, where fully silicided gates offer work functions conducive to p-channel devices independent of polysilicon doping, while partially silicided gates provide work functions appropriate for n-channel devices if they are n-doped. A method for forming a CMOS device using a method compatible with present processing methods for forming both p-channel and n-channel devices, with one of the devices having a fully silicided word line with a first work function and the other of the devices having a partially silicided polysilicon word line with a second work function different from the first work function, would be desirable.